Sulfur is one of the most versatile elements, and it is one of the chemical industry's most widely used raw materials. Still, there are potentially beneficial reactions of sulfur which remain to be developed into practical usage.
For instance, two early references refer to the use of elemental sulfur as a possible reducing agent by which calcium sulfate may be converted to lime and sulfur dioxide. Trautz, Patentschrift No. 356414, states that sulfur vapor may be reacted with calcium sulfate, beginning at a temperature of about 1830.degree. F., to produce lime and sulfur dioxide. To obtain such reaction Trautz caused sulfur vapor to be slowly passed over a combustion boat containing gypsum heated to a temperature of 1832.degree. F. to 2192.degree. F. Horn, U.S. Pat. No. 2,425,740, states that a finely divided mixture of calcium sulfate and elemental sulfur when heated to a temperature of 2400.degree. F. or greater in the presence of excess air, such as in a gas fired rotary kiln, will produce lime and sulfur dioxide.
Theoretically, the reaction between sulfur and calcium sulfate could provide the basis for recovery of sulfur values from gypsum, including that produced in the manufacture of fertilizer grade phosphoric acid by the wet process, where an economical method for recovery of sulfur values from waste gypsum has long been needed.
For each ton of P.sub.2 O.sub.5 produced as H.sub.3 PO.sub.4, about three tons of H.sub.2 SO.sub.4 are consumed and about five tons of wet gypsum (CaSO.sub.4.2H.sub.2 O) are produced as a waste by-product that must be discarded. During 1964, the wet process production of phosphoric acid surpassed two million short tons annually. Current production in the United States is now estimated to exceed five million short tons per year. Worldwide, the growth and production of wet process phosphoric acid has been even more rapid.
According to estimates for current U.S. production, the wet process produces about twenty-five million short tons of waste by-product gypsum annually. Presently, such waste gypsum constitutes an economic and environmental disadvantage of the wet process that has not yet been satisfactorily resolved. First, there is a significant loss of sulfur values to waste gypsum; at current production rates, about 4.75 million tons of sulfur annually. The loss of sulfur values occurs at a time wherein sulfur prices have been increasing greatly, with yet greater price increases expected in the future in view of the large energy requirements of the Frasch Process. The problem is further aggravated in that significant additional processing costs are incurred in the disposal of the waste gypsum. In addition to the manpower and equipment expenditures necessary for disposal, new disposal sites must continually be purchased (approximately ten-fifteen acres a year for a five hundred ton per day wet process phosphoric acid plant) and pumping costs escalate annually as the gypsum waste pile grows higher and/or the distance between the plant site and disposal site increases. Further, disposal of waste gypsum in this manner takes potentially productive land out of circulation and creates an eyesore that may pose environmental problems.
Even though the Trautz chemistry has been known for more than 65 years, and the Horn patent issued 36 years ago, there has been no known commercialization of a process for recovery of sulfur values based on the reductive reaction of calcium sulfate with elemental sulfur.
The difficulties expected to be associated with maintaining effective contact between sulfur, which at the necessary reaction temperature is a gas, and solid calcium sulfate, so that reaction will occur at practical rates in a kiln or other vessel, have in all probability been the prime factor discouraging development of a sulfur value recovery process based upon the laboratory reaction disclosed by Trautz, Patentschrift No. 356414. See for instance Horn, U.S. Pat. No. 2,425,740, who in an attempt to develop a commercial process from such reaction, found that a minimum temperature for reaction of 2400.degree. F. was required and that excess air had to be employed. Horn's disclosure, which teaches the requirement for high temperatures, establishes that the Trautz reaction is not practically adaptable to a commercial process, and so far as is known no further efforts to use sulfur as a reducing agent have been made since Horn.
The problem resides in the conditions for reaction imposed by having to bring a gas and solid into efficient contact for reaction at practical rates. In a reaction kiln solids flow in countercurrent contact to a gaseous atmosphere. The gaseous atmosphere may be the combustion gases by which heat for reaction is supplied to the kiln--as in a gas fired kiln--or may be an inert gas which is flowed through an indirectly heated kiln for purposes of removing gaseous reaction products. Whatever the nature of the gaseous atmosphere, there exists within the kiln a controlled temperature zone wherein reaction occurs. With the exception of that amount of volume within the reaction zone which is occupied by the solids to be reacted, the remaining or free space of the reaction zone is occupied by gas. Any gaseous reagent added to this zone would be expected to rapidly diffuse throughout the gas atmosphere in such free space hence decreasing the concentration of the gaseous reagent at the surface of the solid with which it is to react.
Diffusion of the sulfur vapor throughout the kiln's free space would dictate a need for dispersal of the calcium sulfate into the gas phase in order to effect maximum contact between the reactants. However, such dispersal decreases the effective concentration of CaSO.sub.4 in the reaction zone.
It is known that the rate at which a reaction occurs is proportional to the concentration of the reacting components. For the sulfur - calcium sulfate reaction, a gas-solid reaction, the reaction rate would be proportional to the concentration of gaseous sulfur at the surface of the solid with which it is to react. Increasing the concentration of sulfur vapor at the solid-gas interface would increase the reaction rate.
Although known to be desirable, as yet no means have been devised by which the concentration of a gaseous reagent can be increased at a phase interface by prevention or retardation of the diffusion of that reagent throughout the gaseous phase.